1. Field of the Invention
This invention relates to a recycling fluidization system more particularly to a system in which a gaseous material is subjected to the desired reaction in a downflow type reactor using a finely divided solid material such as a catalyst and a heat medium which recycles through the system.
2. Prior Art
There are known a reaction system in which a solid particle as a catalyst or heat medium is brought into contact with a reactant. A fluidized bed type reactor is a typical example of such reaction system which reactor is classified into one utilizing a dense-phase fluid bed (bubbles-fluid bed) and the other utilizing a high-velocity moving bed. The high-velocity moving fluid bed is used for a reaction in which a solid and gas must come into contact for a short period of time. Currently, there has been mainly employed an upward flow-type high velocity moving bed reactor called "riser" in a fluid catalytic cracking apparatus used for the production of gasoline from a feedstock such as a heavy oil. This reactor is capable of reducing contact time due to an improvement of a catalyst in ability, leading to an enhancement of selectivity of a final product and depression of excessive secondary cracking reaction.
However, a riser-type apparatus accompanies a problem so-called "backmixing" which is a phenomenon that a part of a gaseous material or a solid material flows the reverse direction of the main stream due to gravity acting downwardly on an upwardflow mixture. If such a backmixing of a gaseous material takes place, a part of the gaseous material is removed from a reactor after being reacted incompletely resulting from extremely short contact time and the other part of the gaseous material develops a reaction such as excess cracking due to prolonged contact time, leading to a deterioration of a reaction product. A backmixing of a solid material leads to a reduction of reaction efficiency resulting from a part of the solid material in a deteriorated condition remaining in a reactor for a long period of time.
Recently, in order to evade this problem there has been used a downflow-type reactor as disclosed in Japanese Laid-Open Patent Publication No. 4-261494, U.S. Pat. Nos. 5,462,652 and 4,385,985 and Japanese Patent No. 2,524,425 all of which are intended to shorten contact time and avoid the occurrence of back flow of a gaseous or solid material so as to improve selectivity of the reaction product. However, still some problems are Involved when consideration given to these prior art techniques as a whole a recycling fluidized bed system in which reaction and regeneration are carried out consecutively. More specifically, these prior art techniques can not obtain satisfactory results in view of returning a solid material falling through a downflow reactor to the upstream position where it was in an effective manner for regenerating the solid material to use it again for the reaction.
For instance, it is proposed in Japanese Laid-Open Paten Publication No. 4-261494 to lift up a regenerated catalyst to the inlet of a reactor with a gas used for transportation purposes. However, in this system a blower requires to be increased in capability excessively due to the necessity of using a gas as a medium for lifting up a catalyst other than a combustion gas for regenerating a catalyst, leading to an apparatus complicated in structure which is not preferable in economical point of view. U.S. Pat. No. 5,462,652 has also the same drawback.
U.S. Pat. No, 4,385,985 discloses as to lift up a catalyst using a gas for combustion in a upflow type regenerator. However, the disclosure of this U.S. patent found to be difficult in practical usage with the following reasons. Namely, this prior art is silent as to that pressure loss occurs, compensating for work load in lifting up a catalyst and as a result of this, pressure is increased at the inlet for introducing the air for regeneration, which pressure must be suppressed by static pressure of a dense-phase catalyst bed formed in a stripping device. The pressure at the inlet increased larger than the static pressure of the dense-phase catalyst bed would result in failure of recycling of a catalyst due to backflow of the combustion air toward the stripping device
In order to solve this problem there may be made an attempt to increase the height of the dense-phase bed in the stripping device to increase the static pressure at the stripping device rather than at the inlet. However, this attempt ends in fallure because the distance of lifting up the catalyst must be increased, resulting in an increase of pressure loss involved with lift up of the catalyst. Alternatively, an attempt of increasing the pressure at the stripping device would lead to an increase of pressure of the upper portion of the regenerator which is connected via the reactor with the stripping device. Therefore, the pressure is also increased at the inlet for introducing the air provided on the lower portion of the regenerator. Furthermore, if the blowing rate of combustion air to be used is increased, the catalyst in the regenerator Is decreased in density and pressure loss is also reduced. However, this alternative is economically feasible because the total amount of the air is increased. The above-mentioned Japanese Patent has the same drawback because it proposes to lift up a catalyst to the inlet of a reactor by introducing the recycling air from the lower part of the regenerator.
As described above, it is rather difficult to maintain the pressure balance in a recycling in which solids circulate in a sequence of a reactor, a stripping device, a regenerator and again the reactor while minimizing the amount of the air to be used in a recycling fluid bed reaction system using a downflow reactor. It is more difficult to maintain such a pressure balance in an apparatus in which the recycle ratio of a solid material is large because it requires a large power source to lift up solids.
In a process aiming for improving selectivity of a reaction product for a short contact period, the reaction tends to be effected at a high mixture ratio to maintain a high conversion ratio with contact time kept as short as possible. From view of this background, there has been demanded a system comprising some fluidized or moving bed elements connected in a series which system is simple In structure requiring no air for conveying a solid material and can readily maintain pressure balance to recycle a solid material smoothly. The term "fluidized bed element" used herein designates a series of independent vessel-shape elements constituting a reaction system or apparatus through which a reaction material and the transportation gas for conveying flows consecutively in one direction.